The present invention relates to a perpendicular magnetic storage medium and more particularly to a perpendicular magnetic storage medium used in a form of a card, a tape, a disc or the like, wherein there is formed a perpendicular magnetic anisotropic film made of a sub-oxide of Fe-Co alloy which is suitable for high density recording.
Hitherto, a perpendicular magnetic storage medium is effectively used for a high-density mangetic storage. A magnetic storage medium used for such a purpose utilizes a magnetic thin film which has an easy magnetization axis being perpendicular to the plane of the film. As a magnetic film for the above use, there have been used or researched a thin film of Co-Cr (Co-Cr alloy), Fe.sub.3 O.sub.4 or O.sub.s -.gamma.Fe.sub.2 O.sub.3 produced by sputtering method or vacuum deposition (vacuum evaporation) method, a thin film of barium-ferrite produced by coating method or sputtering method, and the like.
However, the above conventional perpendicular magnetic storage mediums have drawbacks as described hereinafter.
The magnetic thin film of Co-Cr alloy used in the above conventional medium is required to have a structure close to a single crystal. Therefore, a substrate, on which the magnetic thin film is formed, should be heated to higher than 100.degree. C. or frequently higher than 200.degree. C. during the deposition thereon. This requires the use of a heat-resistant substrate and consequently increases the production cost. Further, a film of metal has an intrinsic drawback that it is apt to be worn out.
The magnetic thin film of the metal oxide such as Fe.sub.3 O.sub.4 and O.sub.s -.gamma.Fe.sub.2 O.sub.3 is hard and has a high wear resistance. But, since a substrate for the magnetic thin film of Fe.sub.3 O.sub.4 or O.sub.s -.gamma.Fe.sub.2 O.sub.3 is also required to be heated to higher than 250.degree. C. during the deposition thereon, the production cost is increased like that of the above Co-Cr alloy film. Further, the metal oxide such as O.sub.s -.gamma.Fe.sub.2 O.sub.3 or the like used for the magnetic thin film is sometimes required to be subjected to a reduction process. Still further, the perpendicular magnetic storage medium using these magnetic thin films of the metal oxide has a drawback in that the saturation magnetization (Ms) is low and therefore there cannot be obtained media having high recording and reproducing sensitivity.
The thin film of barium-ferrite obtained by a coating method for the perpendicular magnetic storage medium requires providing barium-ferrite powder having a uniform particle diameter of about 0.1 .mu.m for the film forming thereof. Accordingly, the production cost is increased. Further, because a binder is required to be added in order to form a film, the content of barium-ferrite in the film is lowered due to such addition. This causes a low saturation magnetization (Ms) of the magnetic thin film, and consequently lowers the performance of the magnetic storage medium.
The thin film of barium-ferrite obtained by a sputtering method has a higher saturation magnetization than the film by coating method, but a substrate therefor should be heated to about 500.degree. C. Accordingly, the substrate must be highly heat resistant, and the use of an inexpensive plastic substrate is not allowed.
As an effective means to remove the above drawbacks, there has been proposed a perpendicular magnetic anisotropic film of a sub-oxide of Fe or a sub-oxide of Fe-Co alloy (as disclosed in Japanese Patent Publication No. 162622/1984 or No. 198707/1984, for example). In accordance with such means, a perpendicular magnetic anisotropic film can be formed at a low substrate temperature, and therefore a low heat resistant but inexpensive film can be used as a substrate. In addition, the proposed perpendicular magnetic anisotropic film has a high perpendicular magnetic anisotropy, and accordingly a perpendicular magnetic anisotropic film having a high saturation magnetization (MS) can be obtained. Further, a perpendicular magnetic anisotropic film of such a sub-oxide has a high wear resistance, and also has a high flexibility by virtue of metal contained therein.
However, the recording and reproducing sensitivity and the recording density of the above perpendicular magnetic anisotropic film of a sub oxide of Fe or a sub-oxide of Fe-Co alloy is not so high as, for example, a perpendicular magnetic anisotropic film of Co-Cr alloy. That is, the above perpendicular magnetic anisotropic film of a sub-oxide of Fe or a sub-oxide of Fe-Co presents a normalized output (V) no higher than about 20 .mu. V.sub.o-p /(m/sec)/mm/turn when recording/reproducing is conducted using a Mn-Zn ferrite ring head, while a perpendicular magnetic anisotropic film of Co-Cr alloy presents a normalized output (V) of 40 to 50 .mu. V.sub.o-p /(m/sec)/mm/turn under the same condition. The above normalized output (V) is defined as a quotient obtained by dividing the reproducing output voltage in .mu. V.sub.o-p by the relative head speed in m/sec, the track width in mm and the number of windings in turn.
The present invention was made to solve the above mentioned problem existing in the conventional perpendicular magnetic anisotropic film of a sub oxide of Fe-Co that the recording and reproducing sensitivity thereof is not satisfactory, and an object of the present invention is to provide a perpendicular magnetic storage medium having a perpendicular magnetic anisotropic film, which can be produced easily and economically at a low substrate temperature and which has high wear resistance, and also which has high recording and reproducing sensitivity and high recording density enough to present a normalized output (V) not lower than about 40 .mu. V.sub.o-p /(m/sec)/mm/turn under the aforementioned condition.